Microsoft will begin blocking .msix and .msixbundle file attachments in Outlook Web and the new Outlook Windows client starting in November. These file types are modern Windows installation packages used for specific computer configurations, with .msixbundle serving as a container for multiple packages. The rollout will commence in early November for Exchange Online users, with full availability expected by mid-November. Once implemented, users will be unable to send, receive, open, or download these attachment types.
The security measure is part of Microsoft’s broader initiative to disable features that attackers have exploited in recent years. Most organizations are expected to be unaffected since these file types are rarely used. However, administrators can whitelist the attachments if necessary through their organization’s OwaMailboxPolicy settings. This update follows previous Outlook security enhancements, including the blocking of .library-ms and .search-ms files in June and restrictions on inline SVG images in October.
Source: Read the full story at BleepingComputer
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